The distribution, drug effect, metabolism, and rate of elimination of theophylline will be studied in infants, children and adults to elucidate the mechanisms responsible for age-dependent change in elimination rate. Asthmatic patients will be studied. The rate of excretion of 3 metabolic products will be studied at different ages. Factors affecting distribution, as well as the potential for a liver blood flow effect on elimination will be assessed. The effect of rate of administration on duration of action at different ages will be defined. Thus, a systematic approach to the elucidation of the mechanisms underlying the age-dependent elimination rate of one drug is proposed.